From: Ronan Dunklau <ronan.dunklau@aiven.io>
Date: Fri, 1 Mar 2024 08:57:49 +0100
Subject: [PATCH] Provide a pg_truncate_freespacemap function.

Similar to the pg_truncate_visibilitymap function, this one allows to
avoid downtime to fix an FSM in the case a corruption event happens
---
 contrib/pg_freespacemap/pg_freespacemap.c | 58 +++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/contrib/pg_freespacemap/pg_freespacemap.c b/contrib/pg_freespacemap/pg_freespacemap.c
index b82cab2d97e..fc9ee006793 100644
--- a/contrib/pg_freespacemap/pg_freespacemap.c
+++ b/contrib/pg_freespacemap/pg_freespacemap.c
@@ -9,8 +9,12 @@
 #include "postgres.h"
 
 #include "access/relation.h"
+#include "access/xloginsert.h"
+#include "catalog/storage_xlog.h"
 #include "funcapi.h"
 #include "storage/freespace.h"
+#include "storage/smgr.h"
+#include "utils/rel.h"
 
 PG_MODULE_MAGIC;
 
@@ -20,6 +24,9 @@ PG_MODULE_MAGIC;
  */
 PG_FUNCTION_INFO_V1(pg_freespace);
 
+/* Truncate the free space map, in case of corruption */
+PG_FUNCTION_INFO_V1(pg_truncate_freespace_map);
+
 Datum
 pg_freespace(PG_FUNCTION_ARGS)
 {
@@ -40,3 +47,54 @@ pg_freespace(PG_FUNCTION_ARGS)
 	relation_close(rel, AccessShareLock);
 	PG_RETURN_INT16(freespace);
 }
+
+
+Datum
+pg_truncate_freespace_map(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+	Relation	rel;
+	ForkNumber	fork;
+	BlockNumber block;
+
+	rel = relation_open(relid, AccessExclusiveLock);
+
+	/* Only some relkinds have a visibility map */
+	if (!RELKIND_HAS_TABLE_AM(rel->rd_rel->relkind))
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("relation \"%s\" is of wrong relation kind",
+						RelationGetRelationName(rel)),
+				 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
+
+
+	/* Forcibly reset cached file size */
+	RelationGetSmgr(rel)->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
+
+	/* Just pretend we're going to wipeout the whole rel */
+	block = FreeSpaceMapPrepareTruncateRel(rel, 0);
+
+	if (BlockNumberIsValid(block))
+	{
+		fork = FSM_FORKNUM;
+		smgrtruncate(RelationGetSmgr(rel), &fork, 1, &block);
+	}
+
+	if (RelationNeedsWAL(rel))
+	{
+		xl_smgr_truncate xlrec;
+
+		xlrec.blkno = 0;
+		xlrec.rlocator = rel->rd_locator;
+		xlrec.flags = SMGR_TRUNCATE_FSM;
+
+		XLogBeginInsert();
+		XLogRegisterData((char *) &xlrec, sizeof(xlrec));
+
+		XLogInsert(RM_SMGR_ID, XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
+	}
+
+	relation_close(rel, AccessExclusiveLock);
+
+	PG_RETURN_VOID();
+}
-- 
2.43.2


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